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Matsunaga H.,Kyushu University | Yoshikawa M.,Kyushu University | Yoshikawa M.,Kobe Material Testing Laboratory Co. | Kondo R.,Kyushu University | And 2 more authors.
International Journal of Hydrogen Energy | Year: 2015

Abstract Slow strain rate tensile (SSRT) tests were performed using smooth specimens of two types of steels, the Cr-Mo steel, JIS-SCM435, which has a tempered, martensitic microstructure, and the carbon steel, JIS-SM490B, which has a ferrite/pearlite microstructure. The tests were carried out in nitrogen gas and hydrogen gas, under a pressure of 115 MPa at three different temperatures: 233 K, room temperature and 393 K. In nitrogen gas, these steels exhibited the so-called cup-and-cone fracture at every temperature. In contrast, surface cracking led to a marked reduction in ductility in both steels in hydrogen gas. Nonetheless, even in hydrogen gas, JIS-SCM435 exhibited some reduction of area after the stress-displacement curve reached the tensile strength (TS), whereas JIS-SM490B demonstrated little, if any, necking in hydrogen gas. In addition, tension-compression fatigue testing at room temperature revealed that these steels show no noticeable degradation in fatigue strengths in hydrogen gas, especially in the relatively long-life regime. Considering that there was little or no hydrogen-induced degradation in either the TS or the fatigue strength in JIS-SCM435, it is suggested that the JIS-SCM435 is eligible for safety factor-based fatigue limit design for hydrogen service under pressures up to 115 MPa. Copyright © 2015, Hydrogen Energy Publications, LLC. Source

Yamabe J.,Kyushu University | Awane T.,Kyushu University | Awane T.,Kobe Material Testing Laboratory Co. | Matsuoka S.,Kyushu University
International Journal of Hydrogen Energy | Year: 2015

This study presents a precise hydrogen-barrier mechanism of a newly developed three-layer (alumina/aluminum/ferro-aluminum) aluminum-based coating in high-pressure gaseous hydrogen. After exposure to high-pressure gaseous hydrogen, the hydrogen content of the specimen with a palladium-sputtered aluminum-based coating was the same as that of the specimen with aluminum-based coating, but without palladium. Furthermore, the hydrogen content of the coated specimens increased with a decrease in the specimen size. These results indicate that the hydrogen entered by a diffusion-controlled process. The effective diffusivity of the coated specimen was approximately one thousandth of that of base steel (type 304 stainless). Such excellent resistance could not be obtained with a two-layer coating (alumina/ferro-aluminum). Analysis of local hydrogen concentrations by secondary ion mass spectroscopy demonstrated that the extremely low effective hydrogen diffusivity of the three-layer-coated specimen was attributed to hydrogen trapping at the aluminum-ferro-aluminum interface, and not to the hydrogen-entry obstruction by the aluminum layer. © 2015 Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Source

Yamabe J.,Kyushu University | Awane T.,Kyushu University | Awane T.,Kobe Material Testing Laboratory Co. | Matsuoka S.,Kyushu University
International Journal of Hydrogen Energy | Year: 2015

The apparent hydrogen diffusivity and the saturated hydrogen content of Cr-Mo and Ni-Cr-Mo steels were determined with high-pressure hydrogen gas. Surface effects on hydrogen entry and exit were also investigated by using palladium-coated samples and by diffusion analysis using the finite-element method. Hydrogen contents of hydrogen-exposed cylindrical specimens of various sizes were measured by means of gas chromatography-mass spectrometry to obtain the saturated hydrogen content. The diffusivity was determined by fitting the solution of a diffusion equation to the experimental hydrogen contents determined by desorption at various constant temperatures. In the specimens examined, surface effects were significant at room temperature. The temperature dependences of the diffusivity were reasonably consistent with reference data mainly measured with electrochemical charging. These results were interpreted in terms of hydrogen trapping. Ordinary electrochemical charging represents a more severe condition than exposure to high-pressure hydrogen, for example, at 100MPa. © 2015 Chinese Medical Association Production. Source

Yonezawa T.,Tohoku University | Watanabe M.,Tohoku University | Hashimoto A.,Kobe Material Testing Laboratory Co.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2015

Primary water stress corrosion cracking growth rates (PWSCCGRs) in highly cold-worked thermally treated (TT) Alloy 690 have been recently reported as exhibiting significant heat-to-heat variability. Authors hypothesized that these significant differences could be due to the metallurgical characteristics of each heat. In order to confirm this hypothesis, the effect of fundamental metallurgical characteristics on PWSCCGR measurements in cold-worked TT Alloy 690 has been investigated. The following new observations were made in this study: (1) Microcracks and voids were observed in or near eutectic crystals of grain boundary (GB) M23C6 carbides (primary carbides) after cold rolling, but were not observed before cold rolling. These primary carbides with microcracks and voids were observed in both lightly forged and as-cast and cold-rolled TT Alloy 690 (heat A) as well as in a cold-rolled TT Alloy 690 (heat Y) that simulated the chemical composition and carbide banded structure of the material previously tested by Paraventi and Moshier. However, this was not observed in precipitated (secondary) M23C6 GB carbides in heavily forged and cold-rolled TT Alloy 690 heat A and a cold-rolled commercial TT Alloy 690. (2) From microstructural analyses carried out on the various TT Alloy 690 test materials before and after cold rolling, the amount of eutectic crystals (primary carbides and nitrides) M23C6 and TiN depended on the chemical composition. In particular, the amount of M23C6 depended on the fabrication process. Microcracks and voids in or near the M23C6 and TiN precipitates were generated by the cold rolling process. (3) The PWSCCGRs observed in TT Alloy 690 were different for each heat and fabrication process. The PWSCCGR decreased with increasing Vickers hardness of each heat. However, for the same heats and fabrication processes, the PWSCCGR increased with increasing Vickers hardness due to cold work. Thus, the PWSCCGR must be affected not only by hardness (or equivalently the cold working ratio) but also by grain size, microcracks, and voids of primary M23C6 carbides, etc., which in turn depend on chemical composition and the fabrication process. © 2015, The Author(s). Source

Epoxy resin attached to a fatigue fracture surface of Ti-Al-Nb alloy was removed using a removal method for hardly soluble organic material attached to metallic material, which has been developed by the author. In the removal method process, the epoxy resin attached to the fracture surface was treated with an organic solvent, 'tetrahydrofuran', and cold concentrated sulfuric acid of nearly 100% purity. After the epoxy resin was removed from the fracture surface with the removal method, damage of the microscopic feature of the fracture surface was investigated using a scanning electron microscope (SEM). For the first time, the degree of the removal of the epoxy resin with the method was investigated by energy dispersive X-ray spectroscopy (EDS) in this research. After the removal, no damage of the fracture surface was found with SEM observation. In addition, C K derived from the epoxy resin was not detected with the EDS after removal. The result of the EDS analysis clarified that the epoxy resin was completely removed with the removal method. © 2013 The Author. Source

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